Area Forecast Discussion
Issued by NWS Mt. Holly, NJ

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000 FXUS61 KPHI 210220 AFDPHI Area Forecast Discussion National Weather Service Mount Holly NJ 920 PM EST Sat Jan 20 2018 .SYNOPSIS... High pressure to our south will slowly shift off shore through Sunday. A low pressure system will propagate from the southern Plains to the Great Lakes region Sunday into Tuesday. As it does so, expect first a warm front to arrive in our area Sunday night or Monday, followed by a cold front arriving on Tuesday. High pressure will then build toward our region through Friday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Quiet evening across the area. Ci/Cs clouds overhead with a decent temperature drop in many rural areas. Overnight lows not changed much, just a few tweaks. A zonal flow aloft continues through tonight between a trough well to our north and a weakness aloft near the southeastern states. High pressure at the surface centered near the southeastern states expands northward some tonight while also shifting eastward. Meanwhile, a surface trough in our vicinity should gradually shift south/east and weaken. A northern stream system that tracks into the Canadian Maritimes will try and settle a weak cold front into our area late tonight from the north. Some high level clouds will continue especially for the northern and western areas given the proximity to mid and upper-level jets. There is some weak low-level cold air advection forecast to occur overnight as the flow turns a bit more northwest.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... As a potent closed low begins to eject out into the central and southern Plains during Sunday, some ridging builds toward the western Great Lakes. A more zonal flow aloft however is maintained across much of the East with surface high pressure easing off the southeast U.S. coast with time. A weak surface cold front trying to work down from the north should tend to dissipate with a southern extent. The low-level airmass is forecast to be cooler compared to Saturday, however some low-level warm air advection is forecast to begin developing toward late afternoon as the flow starts to back to the west and west-southwest. We are expecting an increase in cloud cover through the day, and this should tend to lower with time as warming aloft takes place owing to an inversion. It is within the inversion area where the moisture increases and helps to develop lower clouds through the day. Some of the guidance, especially the NAM, looks to fast with the low-level moisture increase and therefore did not rush in the lower clouds as fast. In addition, while blending in continuity for the high temperatures we leaned closer to the warmer GFS/EC MOS as the low clouds indicated by the NAM are thought to be overdone and/or to fast. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Sunday night...We continue to watch where the best isentropic ascent will develop through this period for the potential for light precipitation. At this point, it appears that the favored area will set up to the west of our region, though the GFS does depict a narrow area of cyclogenesis over southeastern PA through this time. For now though, it appears to be a low chance (20 to 40 percent) across our area. If it does happen, then there is also the potential for freezing rain across NW NJ and the southern Poconos, as temperatures will be near freezing for much of the night in this area. Potential remains too low to issue an ice accumulation forecast, but if any precipitation develops we expect it to be light. Monday...We should see a lull in the precipitation between when the initial isentropic lift moves north of the area and when we start to get lift ahead of the approaching cold front. Whether this lull will come Monday afternoon or Monday evening is uncertain as there remain timing differences between different models. Monday night and Tuesday...Ahead of the front, expect a very strong low and mid level southerly jet (60+ kt at 900 mb). I mention this because at this point we expect to have almost negligible instability (not even elevated instability) and a robust temperature inversion, so it is unlikely that the stronger winds will mix down. However, if that changes, it won`t take much mixing to see strong winds at the surface. Given the negligible instability, do not expect any thunderstorms. Precipitable water values could be over one inch, which is certainly above normal for late January, but still considerably less than the 1.46 inches which was observed in the KIAD sounding on the 12th. Additionally, very fast storm motions should limit any flooding threat. As for the front itself, models trended a bit slower with the front. Current forecast is for the front to propagate through the area through the day time hours. The low (and consequently the warm conveyor belt) is expected to be far enough to the NW that we shouldn`t see any precipitation behind the front. Wednesday through Saturday...A large surface high slowly builds east through this period, over our region and eventually off shore. We should start the period below normal thanks to the Tuesday cold front, but could see temperatures moderate as low level flow becomes southwesterly late in the week. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/...
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The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG, KMIV, KACY and surrounding areas. Tonight...VFR with any clouds at or above 15,000 feet. Winds becoming light and variable at most terminals. Sunday...VFR ceilings lowering to between about 4,000-10,000 feet through the day, however some MVFR ceilings may develop especially north and west of PHL. Light and variable winds becoming northwest around 5 knots, then turning southwest in the afternoon. Lower confidence regarding any MVFR or lower ceilings developing. Outlook... Sunday night...MVFR or lower conditions expected across the region mainly due to low ceilings. Some visibility restrictions also possible with light rain. Light (less than 10 kt) southerly or southeasterly winds. Exact timing of when lower clouds will move in is uncertain at this time. Monday...Starting the day with MVFR or lower ceilings. Improvement to VFR is possible in the afternoon, primarily for KPNE, KPHL, KILG, KMIV, and KACY. Improvement is less likely at KRDG, KABE, and KTTN. Southerly or southeasterly winds near or less than 10 kt. Monday night...conditions lowering to IFR or less with low ceilings and visibility restrictions due to rain. Southerly winds of 10 to 15 kt are expected. A period of low level wind shear (LLWS) is likely to develop late Monday night as a strong low level southerly jet develops, though exact timing and location of the jet is uncertain for now. Tuesday...Starting IFR or lower Tuesday morning, but expect quick improvement to VFR conditions behind a cold front expected during the day Tuesday. Also expect an abrupt wind shift from southerly to westerly winds with the cold front. Wind speeds of 10 to 20 kt with higher gusts will be possible especially just behind the cold front. LLWS conditions may linger into Tuesday morning along the coastal plains. Timing of the front, and consequently improving conditions is uncertain at this time. Tuesday night through Thursday...VFR conditions expected. Westerly or northwesterly winds less than 15 kt. High confidence.
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&& .MARINE.. High pressure remains centered to our south through Sunday. With a weak or dissipating cold front settling southward late tonight and Sunday, west-southwest winds become northwest before turning more southwest Sunday afternoon. The conditions are expected to be below Small Craft Advisory criteria through Sunday. Outlook... Sunday night and Monday...Winds and seas are expected to stay below SCA criteria. Monday night into Tuesday morning...SCA conditions for both winds and seas are expected. Expect southerly winds gusting to 30 KT. There is a chance of gale force winds especially on the coastal waters through this period. Late Tuesday through Wednesday...An abrupt shift to westerly winds with gusts above 25 kt is expected behind a cold front moving through late Tuesday. Thursday...Westerly wind gusts near 25 kt are possible through this period, but confidence is below average. && .PHI WATCHES/WARNINGS/ADVISORIES... PA...None. NJ...None. DE...None. MD...None. MARINE...None. && $$ Synopsis...Johnson Near Term...Gorse/Po Short Term...Gorse Long Term...Johnson Aviation...Gorse/Johnson/Po Marine...Gorse/Johnson

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